1. Field of the Invention
The present invention relates to an energy-absorbing member. More particularly, the present invention relates to an energy-absorbing member for an automobile which en-counters lateral compressive loads.
2. Description of the Related Art
An energy-absorbing member is used for improved safety required in case of car crash. Its example is a bumper reinforcement to alleviate damage to the car body at the time of slight collision. An attention is directed to a bumper reinforcement of extruded aluminum alloy for weight reduction. (See Japanese Patent Laid-open Nos. 70688/1995 and 170139/1998.) The bumper reinforcement is a hollow square bar formed by extrusion. It is a so-called crush-able member which, when it receives an external energy by collision, deforms or crushes to absorb crash energy and save other members from damage.
FIG. 1 is a schematic diagram showing how deformation takes place in a bumper reinforcement 1 having a hollow rectangular cross-section (1a and 1b denoting the flanges and 1c and 1d denoting the webs). When the bumper reinforcement 1 receives a compressive force on its outer flange 1a at a right angle, the webs 1c and 1d deform (as indicated by an imaginary line). The energy of load is absorbed in the course of deformation.
Under regulations, a bumper reinforcement should be able to absorb a certain (minimum) amount of energy. If it is so designed as to absorb a large amount of energy, it would be excessively heavy. Therefore, the designer wants a bumper reinforcement to absorb as much energy as necessary without it becoming excessively heavy.
The bumper reinforcement, which is typical of energy-absorbing members, is required to have a large capacity of energy absorption and to be light in weight. To meet this requirement, an attempt has been made to increase the strength of the extruded aluminum alloy for the bumper reinforcement. However, the 7000-series aluminum alloy (Alxe2x80x94Mgxe2x80x94Zn), which is described in the above-cited Japanese patent, is so strong that the web is liable to cracking, with its energy-absorbing capacity decreasing. In other words, the energy-absorbing capacity of extruded aluminum alloy is contradictory to the strength of extruded aluminum alloy for its weight reduction. It has been difficult to cope with this situation by metallurgical means (such as alloy composition and microstructure).
The present invention was completed in view of the foregoing. It is an object of the present invention to provide an automotive energy-absorbing member subject to lateral compressive load, which is made of high-strength Alxe2x80x94Mgxe2x80x94Zn aluminum alloy. This aluminum alloy contributes to strength as well as high energy-absorbing capacity without cracking in case of car crash.
According to the present invention, the energy-absorbing member of extruded aluminum alloy is composed of Mg (0.5-1.6 wt %), Zn (4.0-7.0 wt %), Ti (0.005-0.3 wt %), Cu (0.05-0.6 wt %), and at least one of three elements of Mn (0.2-0.7 wt %), Cr (0.03-0.3 wt %), and Zr (0.05-0.25 wt %), with the remainder being Al and inevitable impurities, and has a hollow cross-section and a fiber structure. In addition, it is finished by overaging treatment. It should preferably have a yield strength greater than 0.7 times the maximum yield strength ("sgr" 0.2 max) that is obtained by aging treatment.
The energy-absorbing member is superior in crushability under lateral pressure. It will find use as automotive parts, such as bumper reinforcement, frame, and door beam, which are subject to compressive load in the lateral direction.